Space solar cell

ABSTRACT

A space solar cell includes a back surface electrode formed on a back surface opposite to a light receiving surface of a semiconductor substrate, and a dielectric layer formed between the back surface electrode and the semiconductor substrate. In the space solar cell, a plurality of openings are formed in the dielectric layer for establishing an electrical connection between the back surface electrode and the semiconductor substrate, and a ratio of an area occupied by the openings relative to an area of the back surface is within a range from 0.25% to 30%.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to Japanese patent application No. HET10(1998)-272678 filed on Sep. 28, 1998 whose priority is claimed under35 USC §119, the disclosure of which is incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a space solar cell, and moreparticularly to a space solar cell (solar cell for space application)such as a space silicon solar cell which has good electrical outputpower characteristics and is suitably used under the environment ofouter space.

2. Description of the Related Art

A silicon solar cell is widely used as a solar cell that converts lightenergy to electric energy. Such a silicon solar cell is used also underthe environment of outer space such as in an artificial satellite.

FIG. 14 shows an example of a conventional silicon solar cell. This isreferred to as BSR (back surface reflector) structure, where an N⁺-typediffusion layer 2 is formed on a light receiving surface located in afront surface of a 200 μm thick P-type silicon substrate 1 by thermaldiffusion of an N-type impurity ions for taking in the carriersgenerated by the light energy, and a light-receiving surface electrode 3is formed in a comb-teeth shape on the N⁺-type diffusion layer 2 fortaking out the generated electricity. Further, the N⁺-type diffusionlayer 2 and the light-receiving surface electrode 3 are covered with ananti-reflection film 4 for reducing a surface reflection of incidentlight.

In addition, a BSR electrode 5 is formed on the back surface of thesilicon substrate 1 for improving the amount of generated carriers byincreasing an optical path length by reflecting a long-wavelength lightthat escapes away from the back surface of the solar cell. Further, aback surface electrode 6 is formed over an entire surface of the BSRelectrode 5 for taking out the generated electricity. In the solar cellof this structure, a conversion efficiency is increased by allowing thelight reaching the back surface of the silicon substrate 1 to bereflected by the BSR electrode 5 to take out an energy of the carriesgenerated around the back surface effectively as an electric power.

A solar cell with further increased conversion efficiency is shown inFIG. 15 and is referred to as an NRS/BSF (non-reflective surface/backsurface field) structure, where a light-receiving surface of a 100 μmthick P-type silicon substrate 1 is formed into a non-reflectionconfiguration 7 with numerous small inverted-pyramid recesses to reducethe surface reflection of solar light by multiple reflection. This isreferred to as “NRS structure”. Also, an N⁺-type diffusion layer 2 isformed on a light receiving surface side of the P-type silicon substrate1, and a front surface oxide film 8 is formed as a front surfacedielectric layer on the N⁺-type diffusion layer 2. A light-receivingsurface electrode 3 having a comb-teeth shape is connected to theN⁺-type diffusion layer 2 via openings formed in the oxide film 8.Further, the oxide film 8 and the light-receiving surface electrode 3are covered with an anti-reflection film 4 for reducing the surfacereflection of incident light.

Further, a P⁺-type diffusion layer 9 is formed on a back surface side ofthe silicon substrate 1 for allowing the carries generated in thesilicon substrate 1 to move towards the N⁺-type diffusion layer (BSFstructure). A back surface oxide film is formed as a back surfacedielectric layer on the P⁺-type diffusion layer 9. The BSR electrode 5and the back surface electrode 6 are electrically connected to theP⁺-type diffusion layer 9 via a plurality of openings 11 formed in theoxide film 10. An internal electric field is formed by the P⁺-typediffusion layer 9, and the carriers generated near the back surface ofthe silicon substrate 1 are accelerated by this electric field, wherebyrecombination of the carriers is prevented and the energy of thecarriers can be taken out effectively as an electric power. In thisstructure, the photosensitivity to a long-wavelength light increases toimprove the conversion efficiency.

Compared with other materials, the solar cell utilizing a siliconsubstrate has a high conversion efficiency and is inexpensive, so thatthere has been a great demand for this type of a solar cell. Especially,in a space solar cell, a further improvement of output power is requiredin recent years and an improvement of an electric output power has beendemanded. Therefore, an improvement of the output power must be achievedalso in the solar cell having the above-mentioned structure.

Thus, in order to achieve an improvement in an output power of a solarcell having a BSR structure, a back surface dielectric layer may beformed on a back surface of a silicon substrate to reduce therecombination, at the back surface, of carriers generated by the lightenergy, whereby the electric output power can be improved. Such a solarcell is disclosed, for example, in Japanese Unexamined PatentPublications No. HEI 04(1992)-274374 and No. HEI 06(1994)-169096.

On the other hand, in the solar cell having an NRS/BSF structure, theback surface dielectric layer is already formed on the back surface ofthe silicon substrate. With respect to increasing the conversionefficiency of the silicon solar cell, “Conference Record” 21th IEEE,Photovoltaic Specialists Conference, Florida, May 1990, pp. 333-335, forexample, proposes a technique in which the substrate includes aplurality of locally-formed P⁺ layers and a silicon oxide film is usedas the back dielectric layer. Also, Japanese Unexamined PatentPublication No. HEI 04(1992)-15963 discloses asolar cell inwhich aspecial arrangement of diffusion layers is provided to increase theconversion efficiency.

In recent years, in addition to the increase of the conversionefficiency of a solar cell, there is also a demand for a space solarcell having good electric output power characteristics in which theradiation hardness is considered so that the solar cell can be usedunder the environment of outer space.

The present invention has been made in view of these circumstances, andthe purpose thereof is to provide a space solar cell that can besuitably used under the environment of outer space by defining anaperture ratio in the back surface dielectric layer, thereby to improvethe electric output power characteristics.

The inventors of the present invention have found out that the loss inthe electric power due to series resistance can be reduced and theelectric output power characteristics can be improved by defining anaperture ratio (area ratio) to be within the range from 0.25 to 30%, theaperture ratio being a ratio of the area occupied by a plurality ofopenings formed in the back surface dielectric layer for establishing anelectrical connection between the semiconductor substrate and the backsurface electrode, relative to the area of the back surface of thesemiconductor substrate. This finding has lead to the present invention.

SUMMARY OF THE INVENTION

The present invention provides a space solar cell comprising a backsurface electrode formed on a back surface opposite to a light receivingsurface of a semiconductor substrate, and a dielectric layer formedbetween the back surface electrode and the semiconductor substrate,wherein a plurality of openings are formed in the dielectric layer forestablishing an electrical connection between the back surface electrodeand the semiconductor substrate, and a ratio of an area occupied by theopenings relative to an area of the back surface is within a range from0.25% to 30%.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the followingdetailed description of preferred embodiments of the invention, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view showing a solar cell having a BSRstructure according to an embodiment of the present invention;

FIG. 2 is a view showing an aperture arrangement pattern;

FIG. 3 is a view showing another aperture arrangement pattern;

FIG. 4 is a view showing characteristics (maximum output power) of thesolar cell having a BSR structure relative to the aperture ratio;

FIG. 5 is a view showing characteristics (solar light absorptivity) ofthe solar cell having a BSR structure relative to the aperture ratio;

FIG. 6 is a view showing characteristics (operating temperature) of thesolar cell having a BSR structure relative to the aperture ratio;

FIG. 7 is a view showing characteristics (maximum output power at theoperating temperature) of the solar cell having a BSR structure relativeto the aperture ratio;

FIG. 8 is a view showing characteristics (maximum output power) of thesolar cell having an NRS/BSF structure relative to the aperture ratio;

FIG. 9 is a view showing characteristics (solar light absorptivity) ofthe solar cell having an NRS/BSF structure relative to the apertureratio;

FIG. 10 is a view showing characteristics (operating temperature) of thesolar cell having anNRS/BSF structure relative to the aperture ratio;

FIG. 11 is a view showing characteristics (maximum output power at theoperating temperature) of the solar cell having an NRS/BSF structurerelative to the aperture ratio;

FIG. 12 is a cross-sectional view showing a solar cell having a BSRstructure with a non-reflection shape;

FIG. 13 is a cross-sectional view showing a solar cell having a BSFRstructure having a front surface oxide film;

FIG. 14 is a cross-sectional view showing a conventional solar cellhaving a BSR structure; and

FIG. 15 is a cross-sectional view showing a conventional solar cellhaving an NRS/BSF structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, a dielectric layer is disposed between a backsurface of a semiconductor substrate and a back surface electrode, and aplurality of openings are formed at an appropriate ratio in thedielectric layer. The area ratio of these openings (aperture ratio)maybe within the range from 0.25 to 30%, preferably with in the rangefrom 10 to 15%, more preferably a value of 12.25%. This reduces theseries resistance and improves the electric output powercharacteristics.

The reason why the area ratio occupied by the openings relative to theback surface of the semiconductor substrate is restricted to be withinthe range of 0.25 to 30% is as follows. If the ratio of the openings islarger than 30%, there will be a smaller effect produced by thedielectric layer for preventing recombination of carriers generated nearthe back surface of the semiconductor substrate; and moreover, since thesolar light absorptivity (α S) increases, the operating temperature willincrease and the output power at the time of actual operation willdecrease. On the other hand, if the ratio is smaller than 0.25%, theseries resistance will increase and the electrical output power willdecrease.

In the above-mentioned construction, the openings are preferablydisposed with equal spacing. In view of facility in manufacturing thesolar cell, the openings preferably have a rectangular shape, morepreferably a square shape. For example, the openings may have a squareshape with a side of about 70 μm and maybe distribute data pitch ofabout 200 μm. Alternatively, the openings may have a square shape with aside of about 30 μm and may be distributed at a pitch of about 300 μm.The openings are not limited to square-shaped dots, and may be circularor polygonal dots. Also, the openings need not be arranged regularly,and may have a random configuration as long as the aperture ratio iswithin the defined range of 0.25 to 30%.

In the present invention, the semiconductor substrate may be an N⁺-P-P⁺junction type semiconductor substrate including an N⁺-type diffusionlayer formed on a light receiving side of a P-type silicon substrate anda P⁺-type diffusion layer formed on a back surface side of the P-typesilicon substrate, or may be a P⁺-N-N⁺ junction type semiconductorsubstrate including a P⁺-type diffusion layer formed on a lightreceiving side of an N-type silicon substrate and an N⁺-type diffusionlayer formed on a back surface side of the N-type silicon substrate.Further, the semiconductor substrate may be an N⁺-P junction typesemiconductor substrate including an N⁺-type diffusion layer formed on alight-receiving side of a P-type silicon substrate, or may be a P⁺-Njunction type semiconductor substrate including a P⁺-type diffusionlayer formed on a light-receiving side of an N-type silicon substrate.

The semiconductor substrate according to the present invention may havea non-reflection configuration (textures) in which the light-receivingsurface is formed to have an uneven shape.

The space solar cell according to the present invention may have any ofaBSR structure, aBSFR (back surface field & reflector) structure, and anNRS/BSF structure.

The dielectric layer may be an oxide film. However, the dielectric layeris not limited to an oxide film alone, and may be a nitride film. If theoxide film is to be used, the oxide film may be an SiO₂ film.

The back surface electrode may include, for example, a three-layeredmetal laminate of Ti—Pd—Ag.

The semiconductor substrate is preferably a silicon substrate in view ofthe photoelectric conversion efficiency and the production costs. Thesemiconductor substrate preferably has a thickness within the range from50 to 250 μm in view of the initial electric output powercharacteristics and the electric output power characteristics afterirradiation.

The semiconductor substrate preferably has a resistivity within therange from 1 to 14 Ωcm in view of the initial electric output powercharacteristics and the electric output power characteristics afterirradiation.

Embodiments

FIG. 1 shows a solar cell having a BSR structure including a backsurface dielectric layer (film) according to an embodiment of thepresent invention. The fundamental structure of this solar cell is anN⁺-P junction type, which is the same as that of the conventional solarcell shown in FIG. 14, wherein like numerals represent like elements inFIG. 14. A silicon substrate 1 having a thickness of 50 μm to 250 μm anda resistivity of 1 Ωcm to 14 Ωcm is used as the semiconductor substrate.For example, a P-type single crystal silicon substrate 1 having a sizeof 36×69 mm, a thickness of 150 μm, and a resistivity of 2 Ωcm is usedas the semiconductor substrate.

In this solar cell, a BSR electrode 5 made of a metal layer such as Alor Au and a back surface electrode 6 made of a metal layer such asTi—Pd—Ag are laminated on a back surface of the silicon substrate 1. Anoxide film 10 made of, for example, SiO₂ and acting as a back surfacedielectric layer is formed between the BSR electrode 5 and the siliconsubstrate 1. In the oxide film 10 are formed a plurality of openings 11which act as contact holes for establishing an electrical connection ofthe silicon substrate 1 with the BSR electrode 5 and the back surfaceelectrode 6.

The oxide film 10 is formed by thermal oxidation or the CVD method. Theopenings 11 are formed in the oxide film 10, for example, by using aphotoetching technique. In these openings 11, the area ratio of theopenings 11 (aperture ratio) relative to the area of the back surface ofthe silicon substrate 1 is set to be within the range of 0.25% to 30%.For example, referring to FIG. 2, the openings 11 are formed to have asquare shape with a 70 μm side and are regularly arranged to bedistributed at a pitch of 200 μm. In this case, the aperture ratio is12.25%, as shown by the following calculation.

(70×70)/(200×200)=0.1225

Further, since the silicon substrate 1 has a size of 36 mm to 69 mm, thedensity of the openings 11 is as follows. In a longitudinal direction ofthe silicon substrate 1 are arranged 345 (=69/0.2) openings, and in alateral direction of the silicon substrate 1 are arranged 180 (=36/0.2)openings. Therefore, the silicon substrate 1 includes a total of 62100(=345×180) openings, whereby the density of the openings is 62100/(36mm×69 mm)=25 openings/mm².

Next, an embodiment will be shown in which the aperture ratio of theback surface dielectric layer in a solar cell of an NRS/BSF structure isoptimized. The structure of the solar cell is an N⁺-P-P⁺ junction typewhich is the same as the one shown in FIG. 15. A P-type single crystalsilicon substrate 1 having a size of 36 mm×69 mm, a thickness of 100 μm,and a resistivity of 2 Ωcm is used as the silicon substrate. A pluralityof openings 11 are formed, for example, by using a photoetchingtechnique in an oxide film 10 serving as a back surface dielectriclayer. These openings 11 are also used as contact holes for obtaining anelectrical connection of a BSR electrode 5 being a metal layer made ofAl and a back surface electrode 6 being a metal layer made of Ti—Pd—Agformed on the oxide film 10, with a P⁺-type diffusion layer 9 on theback surface side of the silicon substrate 1.

Referring to FIG. 3, the openings 11 are formed to have a square shapewith a 30 μm side and are regularly arranged to be distributed at apitch of 300 μm. In this case, the aperture ratio is 1.0%, as shown bythe following calculation.

(30×30)/(300×300)=0.01

Further, since the silicon substrate 1 has a size of 36 mm to 69 mm, thedensity of the openings 11 is as follows. In a longitudinal direction ofthe silicon substrate 1 are arranged 230 (=69/0.3) openings, and in alateral direction of the silicon substrate 1 are arranged 120 (=36/0.3)openings. Therefore, the silicon substrate 1 includes a total of 27600(=230×120) openings, whereby the density of the openings is 27600/(36mm×69 mm)=11.1 openings/mm².

Here, experiments were conducted under an environment of outer space(28° C., AMO) on a solar cell having a BSR structure and a solar cellhaving an NRS/BSF structure in the above embodiments. Tables 1 and 2show experimental data on the electrical output power characteristics(Isc, Voc, FF, Pmax) and the solar light absorptivities as of the solarcell having the BSR structure and the solar cell having the NRS/BSFstructure when the aperture ratio in the oxide film 10 was varied. Here,the dimension of the solar cells is 2 cm×2 cm. The environment of outerspace (28° C., AMO) as used herein refers to the following environment.Also, the electrical output power characteristics were measured underthe following condition.

Under environment of outer space: 28° C., AM0,

Solar light illuminance of 135.3 mW/cm²

Isc: Short circuit current

Voc: Open-circuit voltage

FF: Fill Factor (FF=Pmax/(Isc×Voc))

Pmax: Maximum output power

TABLE 1 Aperture ratio in back Isc Voc Pmax Solar light surface oxidefilm (mA) (mV) FF (mW) absorptivity (αs)  0.25% 161.3 592.2 0.770 73.60.730 12.25% 161.2 593.4 0.785 75.1 0.736 20.25% 161.6 591.8 0.784 75.00.740 30.25% 160.8 587.2 0.779 73.5 0.750 100.00%  159.9 583.5 0.77572.3 0.780

TABLE 2 Aperture ratio in back Isc Voc Pmax Solar light surface oxidefilm (mA) (mV) FF (mW) absorptivity (αs)  0.25% 190.8 629.7 0.776 93.20.860  1.00% 191.0 630.0 0.780 93.8 0.860 12.25% 190.8 629.2 0.779 93.50.863 20.25% 190.5 628.6 0.779 93.3 0.866 30.25% 190.9 625.1 0.770 91.90.869 100.00%  190.5 614.0 0.760 88.9 0.890

FIGS. 4 and 8 show the change of the Pmax (maximum output power)relative to the aperture ratio in the oxide film 10 in the solar cellshaving the respective structures. FIGS. 5 and 9 show the change of thesolar light absorptivities αs relative to the aperture ratio in theoxide film 10 in the solar cells having the respective structures. FIGS.6 and 10 show the change of the operating temperatures Top relative tothe aperture ratio in the oxide film 10 in the solar cells having therespective structures. FIGS. 7 and 11 show the change of the maximumoutput power Pmax^(Top) at the operating temperatures Top relative tothe aperture ratio in the oxide film 10 in the solar cells having therespective structures.

Here, the operating temperature T_(op) is calculated as follows.

T _(op) =[αs×S/(ε_(HF)+ε_(HB))×σ]^(¼)

T_(op): operating temperature (K: absolute temperature)

αs: solar light absorptivity of solar cell

S: solar constant (W/m²)

εHF: hemispherical emissivity of front surface (solar cell) of solarcell array

ε_(HB): hemispherical emissivity of back surface of solar cell array

σ: Stefan-Boltzmann constant (W/m²·K⁴)

As will be apparent from the above experiment results, the improvementin the Pmax is considerable if the aperture ratio in the oxide film 10is 0.25% or more and 30% or less.

The reason why the Pmax of the solar cell increases in accordance withthe improvement in the Voc is that, since the Pmax is represented byPmax=Voc×Isc×FF, the change in the Isc and the FF is comparatively smallif the aperture ratio in the oxide film 10 is 0.25% or more and 30% orless, so that the Pmax is improved in accordance with the Voc.

The reason why the Pmax decreases when the aperture ratio in the oxidefilm 10 is 0.25% or less is that, since the aperture ratio is small, thepath for taking out the generated electricity is long to generate aseries resistance that causes the FF to decrease. Also, the reason whythe effect of improving the Pmax is small when the aperture ratio in theoxide film 10 is 30% or more is that the contact area between thesilicon substrate 1 and the oxide film 10 for preventing therecombination of the carriers generated in the silicon substrate 1 issmall and the Voc improvement obtained as a prevention effect is small.

Also, as will be apparent from FIGS. 5 and 9, in accordance with theincrease of the aperture ratio in the oxide film 10, the solar lightabsorptivity as increases and, as will be apparent from FIGS. 6 and 19,the operating temperature also increases. Therefore, as shown in FIGS. 7and 11, if the aperture ratio in the oxide film 10 is 0.25% or more and30% or less, the solar light absorptivity as and the operatingtemperature do not increase so much, so that the improvement in thePmax^(Top) at the operating temperature is considerable.

Thus, as the operating temperature decreases, the electric output powercharacteristics tend to increase. Since the operating temperature iscorrelated with the solar light absorptivity of the solar cell, thesolar light absorptivity can be decreased and the operating temperaturecan be lowered by forming a back surface dielectric layer on the backsurface of the silicon substrate, whereby the effect of improvement inthe electric output power characteristics increases to make the solarcell suitable for use under the environment of outer space that producesa limited heat-dissipating effect.

Here, the present invention is not limited to the above-mentionedembodiments and numerous changes and modifications can of course be madeto the above-mentioned embodiments within the scope of the presentinvention. Although a P-type silicon substrate is used in thisembodiment, an N-type silicon substrate canbe used as well. If theN-type silicon substrate is used, the diffusion layer on the frontsurface side of the silicon substrate will be P-type and the diffusionlayer on the back surface side of the silicon substrate will be N-type.In other words, a P⁺-N-N⁺ junction type or P⁺-N junction type solar cellis provided. Also, the semiconductor substrate constituting the P-Njunction is not limited to a single crystal silicon substrate alone, sothat polycrystal silicon or other materials can be used as well.

Further, referring to FIG. 12, the solar cell may have a structure inwhich a non-reflection configuration (textures) is formed on thelight-receiving surface of the solar cell (in FIG. 12, a BSR structureis shown). Alternatively, referring to FIG. 13, the solar cell may havea structure in which the light-receiving surface is generally flat (inFIG. 13, a BSFR structure having a front surface oxide film is shown).Further, the BSR electrode on the back surface may be omitted to allowthe back surface electrode to be in direct contact with the dielectriclayer.

In addition, the back surface dielectric layer is not limited to anoxide film alone, and may be a nitride film. Also, the openings in theback surface dielectric film need not be square-shaped dots, and may becircular or polygonal dots. Further, instead of regular arrangement, theopenings may be randomly arranged as long as the aperture ratio iswithin the defined range of 0.25 to 30%. Also, the electrode material isnot limited to the above-mentioned metal alone.

In the above, the aperture ratio of the back surface dielectric layerhas been explained. Hereafter, explanation will be given on thethickness of the semiconductor substrate and the resistivity of thesemiconductor substrate in the case where the aperture ratio in the backsurface dielectric film is set to be 12.25% in order to adapt the solarcell of the present invention particularly for use under the environmentof outer space.

A solar cell to be used under the environment of outer space (spacesolar cell) suffers from a large amount of irradiation in the outerspace, so that the hardness against the radioactive rays is important.In this embodiment, in order to examine the relationship between thethickness of the semiconductor substrate and the radiation hardness, therelationship between the thickness of the semiconductor substrate andthe electrical output power characteristics (Pmax) was examined under acondition with the electron beam radiation amount of 1×10¹⁵ e/cm², whichcorresponds to 10 years in a geostationary orbit under the environmentof outer space, using a solar cell having an NRS/BSF structure and aback surface dielectric layer aperture ratio of 12.25%.

As a result, it is found out that, as shown in Table 3, a thickersemiconductor substrate is more excellent from the view point of initialelectrical output power characteristics, but in view of the electricaloutput power characteristics after the electron irradiation, a thinnersemiconductor substrate is more excellent. Particularly, it is found outthat, if the semiconductor substrate has a thickness of 300 μm or more,the electrical output power characteristics decrease considerably afterthe electron irradiation. Further, if the semiconductor substrate has athickness of 50 μm or less, the yield of good products decreases. Forthis reason, the semiconductor substrate of a solar cell for use underthe environment of outer space preferably has a thickness of 50 to 250μm in view of the endurance against the radioactive rays under theenvironment of outer space.

TABLE 3 Thickness of Electric Property (Pmax) after semiconductorInitial electric irradiation of 1 MeV electrons substrate Property(Pmax) with 1 × 10¹⁵ e/cm² 100 μm 93.8 (mW) 70.3 (mW) 200 μm 97.4 (mW)70.1 (mW) 250 μm 98.5 (mW) 69.9 (mW) 300 μm 99.2 (mW) 68.4 (mW) 350 μm99.3 (mW) 66.5 (mW)

Next, in order to examine the relationship between the resistivity ofthe semiconductor substrate and the radiation hardness, the relationshipbetween the resistivity of the semiconductor substrate and theelectrical output power characteristics (Pmax) was examined under acondition of 1 MeV electron 1×10¹⁵ e/cm²irradiation, which correspondsto 10 years in a geostationary orbit under the environment of outerspace, using a solar cell having a BSFR structure with the semiconductorsubstrate having a thickness of 100 μm and a back surface dielectriclayer aperture ratio of 12.25%.

As a result of this, it has been found out that, as shown in Table 4, alower resistivity is more excellent in view of the initial electricaloutput power characteristics but, in view of the electrical output powercharacteristics after the electron irradiation, the one having aresistivity of 10 Ωcm is more excellent. Also, it has been found outthat the one having a resistivity of 150 Ωcm has both lower initialelectrical output power characteristics and lower electrical outputpower characteristics after the electron irradiation, so that it is notsuitable for use under the environment of outer space. Accordingly,considering the fact that the resistivity of 2 Ωcm is a central value ofa specification of 1 to 3 Ωcm and the resistivity of 10 Ωcm is a centralvalue of a specification of 7 to 14 Ωcm, the resistivity of thesemiconductor substrate is preferably 1 to 14 Ωcm in order to use thesolar cell under the environment of outer space.

TABLE 4 Resistivity of Electric Property (Pmax) after semiconductorInitial electric irradiation of 1 MeV electrons substrate Property(Pmax) with 1 × 10¹⁵ e/cm²  2 Ωcm 80.0 (mW) 53.6 (mW)  10 Ωcm 79.5 (mW)55.7 (mW) 150 Ωcm 76.1 (mW) 54.8 (mW)

It has been found out from the above embodiment that a more excellentradiation hardness is obtained according as the semiconductor substratehas a smaller thickness. Also, the endurance against radioactive rays isespecially excellent if the semiconductor substrate has a resistivityaround 10 Ωcm.

Therefore, as a space solar cell, the thickness of the semiconductorsubstrate is preferably as small as possible in view of the enduranceagainst radioactive rays and the reduction of weight, because a thinnersubstrate leads to reduction of weight of an artificial satellite.

As for the resistivity of the semiconductor substrate, the resistivityis preferably lower in view of the initial electrical output powercharacteristics but is preferably around 10 Ωcm in view of the enduranceagainst irradiation. Therefore, it is necessary to select asemiconductor substrate having a resistivity that can ensure apredetermined minimum electrical output power at the last life stage ofthe artificial satellite, considering the amount of exposure of theartificial satellite to irradiation.

As shown and described above, according to an embodiment of the presentinvention, a plurality of openings for establishing an electricalconnection between the back surface electrode and the semiconductorsubstrate are formed in the dielectric layer which is provided on theback surface of the semiconductor substrate for improvement of theelectrical output power characteristics. By defining the area ratio ofthese openings to be within a predetermined range, the maximum outputpower increases to produce a good output. Therefore, a space solar cellwith improved electric output power characteristics can be obtained.

Further, if the openings are regularly arranged to be distributed withan equal spacing, the effect of providing the dielectric layer will begreat to prevent recombination of the carriers, whereby the output powerwill increase and the conversion efficiency will be improved.

In addition, since the solar light absorptivity can be reduced, theoperating temperature decreases and the maximum output power at theoperating temperature is further improved. Therefore, a solar cellsuitable for use under the environment of outer space can be provided.

Furthermore, a space solar cell having a high output power and anexcellent radiation hardness can be obtained by setting the thickness ofthe semiconductor substrate to be within the range of 50 to 250 μm andsetting the resistivity of the semiconductor substrate to be within therange of 1 to 14 Ωcm.

Although the present invention has fully been described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the invention, they should beconstrued as being included therein.

What is claimed is:
 1. A space solar cell, comprising: a semiconductorsubstrate including a crystalline silicon wafer which functions as anactive layer, said semiconductor substrate including a first diffusionlayer on a light receiving surface thereof; a light receiving sideelectrode formed on at least part of said first diffusion layer of saidsemiconductor substrate; a dielectric layer formed on a back surface ofsaid semiconductor substrate; a back surface electrode formed on thedielectric layer; wherein a plurality of openings are formed in thedielectric layer for establishing an electrical connection between theback surface electrode and the semiconductor substrate, and a ratio ofthe area occupied by the openings relative to the area of the backsurface of the semiconductor substrate is within a range from 10% to 30%in a manner such that recombination of minority carriers on the backsurface of the semiconductor substrate is effectively inhibited; andwherein the back surface of the semiconductor substrate is characterizedby one of: (i) the semiconductor substrate includes a second diffusionlayer on its entire back surface, and (ii) the entire back surface ofthe semiconductor substrate directly contacts said dielectric layer andmaterial in said openings therein with no diffusion layer therebetween.2. A space solar cell according to claim 1, wherein the openings arearranged with equal spacing.
 3. A space solar cell according to claim 1,wherein the semiconductor substrate is an N⁺-P-P⁺ junction typesemiconductor substrate including an N⁺-type diffusion layer formed on alight receiving side of a P-type silicon substrate and a P⁺-typediffusion layer formed on a back surface side of the P-type siliconsubstrate, or is a P⁺-N-N⁺ junction type semiconductor substrateincluding a P⁺-type diffusion layer formed on a light receiving side ofan N-type silicon substrate and an N⁺-type diffusion layer formed on aback surface side of the N-type silicon substrate.
 4. A space solar cellaccording to claim 1, wherein the semiconductor substrate is an N⁺-Pjunction type semiconductor substrate including an N⁺-type diffusionlayer formed on a light-receiving side of a P-type silicon substrate, oris a P⁺-N junction type semiconductor substrate including a P⁺-typediffusion layer formed on a light-receiving side of an N-type siliconsubstrate.
 5. A space solar cell according to claim 3, wherein thelight-receiving surface of the semiconductor substrate is formed in anuneven shape.
 6. A space solar cell according to claim 1, wherein theopenings have a rectangular shape.
 7. A space solar cell according toclaim 6, wherein the openings have a square shape with a side of about70 μm and are distributed at a pitch of about 200 μm.
 8. A space solarcell according to claim 6, wherein the openings have a square shape witha side of about 30 μm and are distributed at a pitch of about 300 μm. 9.A space solar cell according to claim 1, wherein the space solar cellhas one of a BSR structure, a BSFR structure, and an NRS/BSF structure.10. A space solar cell according to claim 1, wherein the back surfaceelectrode comprises a three-layered metal laminate of Ti—Pd—Ag.
 11. Aspace solar cell according to claim 1, wherein the semiconductorsubstrate has a thickness of 50 to 250 μm and a resistivity of 1 to 14Ωcm.
 12. The solar cell of claim 1, wherein the range is from 10% to15%.
 13. A space solar cell, comprising: a back surface electrode formedon a back surface opposite to a light receiving surface of asemiconductor substrate; and a dielectric layer formed between the backsurface electrode and the semiconductor substrate, wherein a pluralityof openings are formed in the dielectric layer for establishing anelectrical connection between the back surface electrode and thesemiconductor substrate, and a ratio of the area occupied by theopenings relative to the area of the back surface of the semiconductorsubstrate is within a range from 10% to 30%.
 14. The solar cell of claim13, wherein the range is from 10% to 15%.
 15. The solar cell of claim13, wherein the back surface of the semiconductor substrate ischaracterized by one of: (i) the semiconductor substrate includes asecond diffusion layer on its entire back surface, and (ii) the entireback surface of the semiconductor substrate directly contacts saiddielectric layer and material in said openings therein with no diffusionlayer therebetween.
 16. The solar cell of claim 13, further comprising asecond diffusion layer provided across the back surface of saidsemiconductor substrate.